The present invention relates to a shear foil for dry shavers, and more particularly to a shear foil which is provided with holes throughout.
Dry shavers having vibrating cutting systems are provided with shear foils which are bent substantially in a semi-cylindrical configuration over the cutting edges of the cutting system which is reciprocated in a straight-line movement beneath the shear foil so that the cutting edges of the system cut off the beard hair that projects through holes in the shear foil when the latter is placed against the face of a user. In some instances the cutting system is rotated, rather than reciprocated in a straight line.
One of the factors which influences the quality of the shave that can be obtained with a dry shaver of this kind, is the perforation of the shear foil itself. Many different types of perforations have been tried in an attempt to arrive at an optimum type of perforation which will give close and comfortable shaves. It is known to provide shear foils with circular holes, with quadratic holes, with honeycomb-shaped holes, with rectangular and with eliptical holes. One thing that all shear foils of the prior art have in common despite the differences in the holes is that the shear foil should have the least possible thickness so that the cutting edges of the cutting system should be located as close as possible to the face of the user and therefore cut off the beard hair as short as possible. It has also been found desirable in this connection to provide in the shear foil, particularly in the region of the central portion thereof, the ratio of the total cross-sectional area of the holes to the ratio of the remaining unperforated part of the shear foil body, as large as possible. On the other hand, the ratio of holes to unperforated perforated portion of the shear foil body cannot be made to exceed a certain limit, because otherwise the skin of the user might in part be pressed through the holes and come in contact with the cutting edges of the cutting system which would lead to possible injury and certainly to discomfort.
However, it is not only the size and geometric configuration of the holes in the shear foil which are important in terms of obtaining a good and close shave. Another factor to be considered in the configuration of the holes in direction normal to the surface of the shear foil. To have a comfortable shave which is not painful to the skin, the holes in the shear foil must be rounded at the side of the shear foil which contacts the skin of a user. This assures that the beard hairs will readily enter the holes when the shear foil is made to move over the surface of the skin of the user, and that the feel of the shear foil on the skin is smooth and soft.
Special requirements are also made of the holes at the side of the shear foil which faces the cutting edges of the cutting system. For example, if the shear foil were to be completely planar, then the punching of holes in direction strictly normal to the shear foil would, when the shear foil is in contact with the cutting edges of the cutting system, cause the cutting edges to be in contact with the unperforated portions of the shear foil intermediate the holes. This results in a relatively high amount of frictional heating which is found uncomfortable on the skin of a user. For this reason it has been attempted to make the unperforated portions of the shear foil intermediate the holes-- which are formed in rows-- curved so that the contact between these unperforated portions and the cutting edges is as small as possible, thus reducing the frictional heating Generally speaking, it is now industry practice to so configurate the contact face that it does not exceed a third of the total surface of the foil.
When such shear foils are produced, where the shear foil portions bounding the holes on the inwardly directed side of the shear foil are raised with respect to the surrounding non-perforated part of the shear foil, it must be assured that the raised part is not high enough to prevent the beard hair from being cut as close as possible to the skin. On the other hand, if the raised part is made relatively low, this often results in a weak marginal portion of the shear foil because the central part of the shear foil and the marginal portion are produced in a unitary operation. For example, if the shear foil is produced by drilling, stamping or the like, and if the starting material is a non-perforated steel foil whose strength would be sufficient to withstand the maximum expected textile and pressure forces to which it would later be subjected as a shear foil, then the raised portions bounding the openings would be too high and the beard hair would not be cut close enough to the skin. On the other hand, if the steel foil used as a starting material is thin enough to assure that the beard hair is cut off close to the skin, then the marginal portion of the shear foil is generally too weak to be able to withstand the forces that act upon the shear foil in use. To overcome this problem it is customary to reinforce the marginal portions by separate reinforcing elements.
In the case of the drilled, stamped or otherwise produced steel foils which are converted into shear foils, there is the disadvantage that relatively complicated hole configurations are difficult to produce. As a general rule, foils of this type are usually provided with holes of cylindrical configuration whose edges are more or less sharp. This means that if the user happens to have a tender skin, there will be substantial friction upon it by the small sharp edges and great discomfort will be caused. Moreover, steel tends to readily corrode in moisture, for example near the sea shore or in the tropics.
Present-day shear foils are usually produced galvanoplastically or by means of etching. A particularly advantageous shear foil material has been found to be nickel. However, in the case of shear foils which are produced galvanoplastically analogous problems as in the case of steel foils are found. Here, again, a compromise must be made between the thickness in the central region of the foil and the thickness at the foil margin. In addition, when a shear foil is produced galvanoplastically, there is the further fact that due to distortions of the electric field used in producing the shear foil the deposition of material is different at the perforated parts of the foil than at the non-perforated parts. This means that marginal portions of shear foils which are produced galvanoplastically require as a rule reinforcing element to make them strong enough so as to prevent the shear foil from becoming destroyed.
It had also been proposed, in order to prevent destruction of the shear foil at the transitional zone which connects the central perforated portion to the marginal portions, and in which particularly high stresses develop, to decrease the holes in the outer portions of the apertured part while leaving identical center-to-center spacing. However, experience has shown that if resort is had to this proposal, the marginal portions are still not strong enough to withstand the stresses that act upon them and must still be provided with reinforcing elements.
A further proposal suggests that the central portion have the holes distributed according to a constant grid, whereas the size of the holes in the marginal portion decreases towards the edge in direction transverse to the direction of movement of the cutting system. The ratio of the hole size to the foil thickness is substantially constant in the marginal zone. However, it has been found that this also is not a construction in which separate reinforcing elements can be omitted.
Further constructions have proposed holes having different sizes, i.e. the size of the holes in direction transverse to the movement of the cutting system varies. The purpose of these constructions is to accommodate them to beard hairs of different thickness. However, these foils also cannot do without separate reinforcements for the marginal mounting portions.